1. Field of the Invention
This invention relates to monitoring the residue on a surface during a chemical treatment of the surface, such as cleaning and drying processes during the manufacture of ICs, MEMS and other micro devices and more specifically to a micro sensor for monitoring residue on and in dielectric films and cells on the surface of the sensor.
2. Description of the Related Art
A major challenge in manufacturing of the micro and nano devices is the cleaning and drying of very small “micro features”. These micro features are fabricated in various processing steps and can be very small voids such as gaps, holes, vias or trenches that are intentionally etched, dielectric surfaces, pores in the dielectric surface material or possibly cells (biologic or other) on the surface. Cleaning and drying occur repeatedly during the processing chain and are responsible for a significant part of the total processing time and for the consumption of much of the water, chemicals and energy.
In Integrated Circuits, MEMS and other micro device manufacturing well controlled cleaning and drying of surfaces and micro features are essential to avoid deformation of layers and improper adhesion of moving parts. Improper cleaning and drying would have a significant effect on manufacturing yield and device performance and reliability in both semiconductor and MEMS fabrication. Over-cleaning, over-rinsing or over-drying results in excessive use of chemicals, water and energy and also increases cycle time and potentially causes yield loss. Therefore, there is a strong economic and environmental incentive to use a process that is “just good enough”.
The surfaces and fine structures left behind after processes such as etching, deposition, and patterning, need to be cleaned and the reaction by-products need to be removed often down to trace levels. This usually involves three steps: 1) application of a cleaning solution; 2) rinsing and/or purging using ultra pure water or other rinsing solutions; and 3) drying by removing and purging the traces of any solvents used during rinsing. Due to the undesirable surface tension associated with aqueous chemicals and non-wetting nature of most future dielectrics, industry is pursing the development of) processes based on supercritical fluids such as supercritical carbon dioxide for cleaning and pattern development. Measurement of cleanliness under these processing conditions is very critical.
Cleaning, rinsing, and subsequent drying processes are often performed and controlled almost “blindly” and based on trial and error or past experience. The way these processes are monitored and controlled presently is based on ex-situ testing of wafer, chips, or structures. Within the process tool, fixed recipes are provided by tools and process suppliers. Run-by-run adjustments or control are based on external and delayed information on product performance or product yields. The key reason for this inefficient and costly approach is that no sensors or techniques are available to measure the cleanliness and monitor the removal of impurities from micro features—to measure cleanliness where it actually counts. The sensors that are currently available are used in the fabs to monitor the conditions of fluid inside the process vessels and tanks, but far away from the inside of micro features (that is what needs to be monitored; it is also the bottleneck of cleaning and drying). The present monitoring techniques and devices do not provide realistic and accurate information on the cleanliness and condition of micro features.
Industry currently works around this problem while waiting for a solution; the process condition and cleaning and drying are often set with very large factors of safety (over-cleaning and over-rinsing). Large quantities of water and other chemicals are used (much more than what is really needed). This results in wasted chemicals, increased process time, lowered throughput, increased cost, and it causes reliability issues because of lack of process control.
K. Romero et al “In-situ analysis of wafer surface and deep trench rinse,” Cleaning Technology in Semiconductor Device Manufacturing VI, The Electrochemical Society, 2000 propose a device for monitoring the process in-situ for high aspect ratio trenches. The trench device comprises a pair of conducting electrodes (Poly-Si) sandwiched between dielectric (SiO2) layers on opposite sides of a trench. An impedance analyzer applies a measurement voltage to the electrodes, which carry the measurement signal (voltage and current) to the trench. The impedance analyzer measures the impedance between its two terminals (ratio of voltage and current and the phase difference between the voltage and current).
For the sensor to be useful as a monitor of the fluid in the micro feature, the total parasitic capacitance between the electrodes and the substrate and/or fluid must be sufficiently small to allow an electrical measurement of the total impedance between the electrodes to resolve the solution resistance Rsol′n and/or the interface double layer capacitance Cd1. If the parasitic capacitance dominates the total electrical response, then the circuit will not have a good signal to noise ratio and the sensor will not be very sensitive. In the paper by Romero et al., the parasitic capacitance was found to dominate the solution resistance. At the parasitic capacitance measured (88 pF), the equivalent circuit calculation predicts no discernable impedance variation between highest and lowest trench resistances. The full ionic concentration range was not experimentally resolvable in comparison to electronic noise.
Romero's trench device does not address in-situ monitoring of dielectric surfaces and does resolve the problem of parasitic capacitance.